In the process of drilling wells for oil and gas production, maintaining a stable borehole is of primary importance. Borehole stability is dependent on mechanical interaction between the drill string and borehole walls, as well as mechanical and chemical interactions between the drilling mud and the walls of the borehole. It is important for the drilling fluid to allow minimal hydration and dispersion of shale cuttings to prevent excessive solids buildup in the mud which can significantly increase drilling costs.
One of the most significant causes of wellbore instability is shale hydration. Shales are fine-grained, sedimentary rocks containing significant amounts of clay minerals. Shales containing high concentrations of montmorillonite, a high swelling and dispersive clay, readily swell and disperse when contacted by water, resulting in excessive solids buildup in the drilling mud. To minimize shale hydration, it is desired to utilize a drilling fluid that exhibits minimal reactivity with shale formations.
High molecular weight acrylamide/acrylate copolymer is commercially available and is well known for its ability to impart borehole stability by inhibiting shale hydration. The polymer is believed to adsorb on the positive-charged edges of the clay platelets where it acts as a protective colloid to reduce the rate of hydration of the clay without altering the internal forces holding the individual platelets together. The polymer is especially effective when it is used in conjunction with a source of potassium ions such as KCl. Sodium ions are typically found between the unit layers of clay where they are weakly held and tend to promote dispersion and swelling. Potassium ions are preferentially exchanged for the sodium ions held between the unit layers. The potassium ions combine a low hydrational energy with a small size that allows minimal hydration of the shale. The potassium ions are small enough to enter between the unit layers of clays in shales that are not highly hydrated where they are tightly held and can limit further hydration.
Although acrylamide/acrylate copolymers are readily available from various commercial suppliers, there is still a desire for a product with improved performance. Of particular interest is a chemical that would provide improved shale inhibition without increasing the viscosity of the base mud.
J. J. Sheu et al., Society of Petroleum Engineers (SPE) 18033 (1988), describes the design of improved shale stabilizing polymers by converting 20% of the carboxylate functionality of partially hydrolyzed polyacrylamide (50% carboxylate/50% amide) into alcohol groups. J. A. Wingrave et al., SPE 16687 (1987), describes quantitative shale stabilization testing. High molecular weight polymers including polyacrylamides are said to be effective when combined with potassium ions. B. C. Chesser International Association of Drilling Contracters/Society of Petroleum Engineers (IADC/SPC) 14757 (1986), describes the techniques of using high molecular weight organic polymers, specifically partially hydrolyzed polyacrylamide for shale stabilization. G. M. Bol, IADC/SPE 14802 (1986), describes the effects of various polymers and salts on borehole and cutting stability in water-based drilling fluids. Polymers tested included polyacrylamide and biopolymers such as cellulose-based polymers, xanthan gum and succinoglycan. Ronald P. Steiger, Journal of Petroleum Technology, pp. 1661-1670, August 1982, describes relatively simple potassium/polymer drilling fluid systems, comprising a high molecular weight partially hydrolyzed polyacrylamide. R. K. Clark et al., Journal of Petroleum Technology, pp. 719-727, June 1976, describes a polyacrylamide/potassium chloride mud for drilling water-sensitive shales.
Water soluble tetrapolymers of N-vinylpyrrolidone, acrylamide, acrylic acid salt and an N-alkylacrylamide are disclosed in U.S. Pat. No. 4,663,408 to Schultz et al. The polymer is prepared by terpolymerization of the acrylamide, N-substituted acrylamide and the pyrrolidone by free radicals in an aqueous medium followed by base catalyzed hydrolysis to partially convert the amide radicals to acrylate salt radicals. The tetrapolymer is said to be useful as aqueous or brine solution viscosifiers.
U.S. Pat. No. 4,673,716 to Siano discloses water soluble copolymers of acrylamide, oil soluble higher alkylacrylamide, and alkali metal acrylate to provide efficient viscosification of water or brine. The synthesis of the polymer relies on the complete solubilization of the water insoluble monomer into an aqueous solution of the water soluble monomers by means of a water soluble surfactant. The polymer may also be post-treated by the addition of base in order to produce a partial hydrolysis of the acrylamide to give a metal salt of acrylic acid.
U.S. Pat. No. 4,719,021 to Branch, III discloses shale stabilizing drilling fluids containing polyvalent metal/guanidine complexes.
U.S. Pat. No. 4,757,862 to Naimen et al. discloses water-soluble copolymers of a vinyl carboxyamide and a hydroxy-containing vinyl carboxylic ester which are essentially free of cross-linking.
U.S. Pat. No. 4,921,903 to Fong, which is hereby incorporated herein by reference, discloses a process for making a random transamidated acrylamide terpolymer as a water-in-oil emulsion.
Gray et al., Composition and Properties of Oil Well Drilling Fluids, 4th Ed., Gulf Publishing Co., Houston, Tex. (1980), pp. 414-421, describes the effect of mud properties on drilling rate. Density is the most important property, but viscosity is also known to have a material influence. The lower the density and viscosity, the higher the drilling rate. Therefore, it is important that any shale stabilizer additive not result in excessive viscosification of the drilling fluid.